This invention is directed to apparatus for measuring time jitter produced by data transmission systems and in particular to apparatus for measuring jitter in a system during transmission.
Time or phase jitter measurement techniques have been used for many years. These methods, which require the use of a jitter free reference clock, are particularly convenient for laboratory measurements and factory acceptance testing, but are not appropriate for field measurements where the distance between consecutive sections prohibits the economic realization of a separate carrier for the jitter free reference clock. Examples of jitter measurement apparatus using a jitter free reference clock are described in U.S. Pat. No. 3,711,773 which issued on Jan. 16, 1973 to Hekimian et al, U.S. Pat. No. 3,895,186 which issued on July 15, 1975 to Yoshida et al, and U.S. Pat. No. 3,916,307 which issued on Oct. 28, 1975 to Hekimian. It has been found however, that it is difficult to build a jitter free reference, which for accurate, reliable measurement purposes should have a jitter which is about 10 times smaller than the clock jitter to be measured. This difficulty has been found to be especially true for high speed systems having a transmission rate of 90 Mb/sec or higher rates. The need for in-service time jitter determining apparatus is especially felt in digital microwave, digital cable, satellite and regenerative optical fiber link systems particularly of the multi-hop type. In these systems, field troubleshooting is very expensive since, after installation, it is possible that equipment misalignment, temperature variations, humidity, any type of noise or interference, aging of components or any of a number of other causes may be such that the end-to-end system jitter becomes excessive. Jitter might also become excessive only during certain parts of a day while at other times it may not noticeably degrade the overall system.